Notch proteins are transmembrane receptor proteins. There are four such notch receptors in mammals. During receptor maturation, the ectodomains of mammalian notch receptors are cleaved at a S1 site by a furin-like protease, yielding an extracellular subunit and a transmembrane subunit that are held together by a heterodimerization (HD) domain. The part of HD domain associated with extracellular subunit is referred to as HD-N, and the other part of HD, extracellular moiety of transmembrane subunit, referred to as HD-C. The extracellular subunit contains a large epidermal growth factor (EGF)-like repeat region and three Lin12 repeats. Ligand binding of the EGF-repeat regions induces a proteolytic cleavage by ADAM-type metalloprotease at the S2 site within the HD-C domain, which triggered subsequent cleavage by γ-secretase at site S3 releases the intracellular part of notch from the membrane, allowing it to move into the nucleus and regulate gene transcription. (Gordon, W. R., et.al, Nature Structural &Molecular Biology, 2007, volume 14, 295-300).
Before ligand induced activation, notch is maintained in a resting metalloprotease-resistant confirmation by a conserved negative regulation region (NRR), which consists of the three Lin12 repeats and the HD domain. (Vardar et al., Biochemistry 2003, 41: 7061-7067; Sanchez-Irizarry et al., Mol. Cell. Biol. 2004, 24: 9265-9273; Gordon, W. R., et.al, Nature Structural &Molecular Biology, 2007, volume 14, 295-300). NRR of the notch proteins is also sometimes defined as only consisting the Lin12 repeats and the N terminal HD domain (HD-N) after proteolytic cleavage at the S1 site. (Weng, A. P., et. al, Science, 2004, 9265-9273.) NRR domain prevents the ligand-independent proteolysis of the notch pathway.
The Notch pathways functions during diverse developmental and physiological process including those affecting neurogenesis in flies and vertebrates. In general, notch signaling is involved in lateral inhibition, lineage decisions, and the establishment of boundaries between groups of cells. (Bray, S. J., Nature Reviews, 2006, 678-688).
However, notch activities are also associated with a variety of human diseases, including cancer. For example, mutations of notch1 were detected in more than 50% of T-cell acute lymphoblastic leukemia. (Radtke, F, Nature Review, Cancer, 2003, 756-767). There is a need in identifying therapeutic agents that regulate the notch-1 signaling pathway for the use of treating cancer.